University of Pennsylvania Medical Center Guidelines for Antibiotic Use

Bauer-Kirby Disk Diffusion

This is one of the more commonly used methods of antimicrobial susceptibility testing. In this test, small filter paper disks (6 mm) impregnated with a standard amount of antibiotic are placed onto an agar plate to which bacteria have been swabbed. The plates are incubated overnight, and the zone of inhibition of bacterial growth is used as a measure of susceptibility (see below). Large zones of inhibition indicate that the organism is susceptible, while small or no zone of inhibition indicateresistance. An interpretation of intermediate is given for zones which fall between the accepted cutoffs for the other interpretations.

An example of a plate from a disc diffusion study. Note the varying sizes of the inhibitory zone present around the different disks. Each disk is impregnated with a different antibiotic agent. Interpretation of the test requires comparison of the measured zone diameter with the accepted cutoff values for each antibiotic/organism pair.

A closer view of the inhibition zones.

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